The present invention relates generally to field emission tips, and more particularly to field emission tips upon which a metal adsorbate has been selectively deposited.
Field emission sources capable of emitting highly focused electron beams, such as are used in scanning electron microscopes, are commonly made from a single tungsten crystal. As shown in the article "Angular Confinement of Field Electron and Ion Emission," by L. W. Swanson and L. C. Crouser, Journal of Applied Physics, Vol. 40, No. 12, Nov 1969, pp 4741-4749, coating the emission tip with a metal adsorbate, such as zirconium, will greatly increase the emission from the tip. Because such a coating process requires a very low vapor pressure environment, however, such coatings are not commercially feasible. Additional problems from such coatings arise because certain planes of a body-centered cubic crystal, such as tungsten, have a lower surface density of atoms than others. The lower work function of these lower density planes results in greater adsorption on these planes than on the more closely packed planes. If these low density surfaces are not the ones from which emission is desired, the quality of results is diminished.
A second procedure for improving the emission characteristics of a tungsten field emitter, also described in the above article, is by the use of thermal field buildup. Procedures have been developed by which faceting of selected planes may be produced with a resulting reduction in both angular dispersion of emissions from the tip and reduced beam voltage necessary for emission. However, such emitters are highly susceptible to temperature changes, and tend to be relatively unstable. Additionally, they are very susceptible to ion bombardment and thus require a very high vacuum for effective operation.
It is desired to have a coated tip which can be produced and operated at a commercially feasible vapor pressure, which has improved stability characteristics, and which has both reduced angular beam dispersion and increased brightness over field emitters currently available.